CBBC is facing the third and most intense marine heatwave the UK has seen this year, and Dr Ségolène Berthou says the ocean did not have enough time to cool down between the two land heatwaves. The Met Office says UK waters are now in an extreme marine heatwave, with temperatures on average 2C warmer than usual.
Some locations are already reaching severe conditions, where the water is 4-5C warmer than usual. Berthou said: “There’s no sign of an end to it.” For anyone tied to marine life, that is the practical takeaway: the heat is not a brief spike, but a sustained stretch that can shift what survives in the water.
Met Office readings in UK waters
The third heatwave of the summer arrived in the UK this week, and temperatures in the UK are on track to exceed 30C for up to 10 consecutive days. The marine heatwave developed rapidly after last month’s heat dome, which drove ocean temperatures up just as most of Europe sweltered in its worst ever heatwave. Oceans absorb more than 90% of the excess energy in the Earth system, and that energy is primarily caused by burning fossil fuels.
The current pattern is severe because the sea never got the reset it usually needs. Berthou said the ocean had no time to cool between the two land heatwaves, which left marine waters starting from a warmer base and rising again before recovery could happen.
Copernicus records and El Niño
The Copernicus Climate Change Service and the Copernicus Marine Service said global sea surface temperatures have surpassed the previous records for this time of year, set in 2023 and 2024. Those temperatures had been anticipated as El Niño conditions developed, with scientists forecasting the phenomenon to be the strongest in decades. António Guterres called the arrival of El Niño conditions the “urgent climate warning it is.”
The complication is that the record warmth was expected, but that does not make it harmless. Global sea temperatures sitting at record levels for this point in the year still raise the odds of marine stress when UK waters are already running hot.
Fish, shellfish and new species
Prof John Pinnegar, principal scientist and lead adviser at the Centre for Environment, Fisheries and Aquaculture Science, said extreme marine heatwave conditions can result in “mass-mortality events” for some marine species. He also said they can alter the distribution of commercially important fish and shellfish, which matters for the people who rely on predictable seasonal waters.
Pinnegar added: “Prolonged periods of elevated sea water temperatures can also encourage new species to visit UK waters, establish new populations, potentially shaking up UK ecosystems.” Record numbers of octopuses were found off the south-west coast of England last year, and a record 100 tonnes of octopus was sold in one day at Brixham market last month. If this warm spell persists, the immediate question is which species are hit first: the ones that cannot move, or the ones that move in and stay.
